Neutron detectors are devices for detecting the presence of thermal neutrons, which are free neutrons (i.e., not bound within an atomic nucleus) that are generated in relatively large numbers by certain hazardous materials (e.g., materials that can be used to produce nuclear weapons). As such, neutron detectors are typically used to detect the presence of hazardous materials by identifying an unusually high number of thermal neutrons in the environment surrounding the hazardous materials.
Because neutrons carry no electrical charge, neutron detection requires the use of a “neutron capture” material (i.e., a material with a high neutron capture cross-section) that “converts” neutrons into charged particles that can then be detected using an electrical sensor. This conversion process typically involves an absorptive reaction in which neutrons are absorbed by the neutron capture material, which then react to the absorption event by emitting high energy ionized particles. Conventional neutron capture materials include Helium-3, Lithium-6, Boron-10 and Uranium-235.
Early tube-type neutron detectors typically use Helium-3 gas as a neutron capture material, and can achieve very high thermal neutron detection efficiencies (e.g., a 2″ diameter tube filled with 10 atm He3 gas are capable of approximately 80% detection efficiency). However, tube-type neutron detectors are expensive to produce, and for various reasons (e.g., gas leakage), are normally operated at lower gas pressures, which greatly reduces the detector efficiency.
More recently, solid-state semiconductor-based neutron detectors have been developed that typically operate by coating a silicon photodiode structure (p-n junction) with a solid material having high neutron capture cross-section, such as Boron-10. Incident neutrons are converted to ionizing radiation (alpha particles) in the boron-10 layer, which in turn generate electron-hole pairs in the silicon that are separated and collected by the p-n junction. A problem with such conventional solid-state neutron detectors is that the vapor deposited silicon layers used to produce conventional solid state neutron detectors require expensive semiconductor processing techniques.
What is needed is a bulk manufacturing method for producing low-cost neutron detectors that can be utilized in certain applications, such as distributed item level tagging of packages during shipping to detect hazardous radioactive materials.